Of the 350 students in a hall, 20% are girls. then,50 more girls entered the hall. now, what percent of students in the hall are girls

Original = 350 students 20% are girls --> 350*0.20 = 70 girls.
New = Since it is adding 50 girls that means the overall population has to add 50 as well --> 350 + 50 = 400 students. 70 girls originally + 50 extra girls = 120 girls. 120 divided by 400 = 30% of the students are girls